If you didn't develop in that timeframe you might not get the difference between then and today. It takes forever to look things up so you end up memorizing instead. And it isn't a big deal because the number of things needed to be a successful software developer wasn't that great. To be successful and finish projects more or less on time, memorization was needed.

Edit: For example, for a different project I knew the hex codes for every 8051 instruction. This was necessary to be successful debugging using a logic analyzer. Looking up each op-code would be prohibitively slow.